Paper is an attractive substrate for sustainable and scalable organic electronics; however, its intrinsically insulating nature, the absence of continuous electronic pathways, and the lack of control over mixed ionic–electronic transport have limited its use in electrochemical devices. Here, we nanoengineer cellulose fibers by introducing cationic charges to facilitate a high specific surface area accessible for the adsorption of functional components. We further speed up the diffusion-controlled adsorption through controlled partial fibrillation of the fibers. The combined cationic charge and high surface area enabled high adsorption of the conducting polymer PEDOT:PSS (poly(3,4-ethylenedioxythiophene):polystyrene sulfonate) throughout the internal nanostructure of the fiber wall. The modified fibers were then rapidly transformed to mechanically robust, electrically conductive papers using a conventional papermaking methodology. Post-treatment of papers containing 30 wt% PEDOT:PSS resulted in excellent charge transport and a conductivity as high as 13 S cm−1. Furthermore, electrochemical impedance spectroscopy of wet papers confirmed effective mixed ionic–electronic transport. Finally, to demonstrate the possibilities of the electroactive paper, we integrated the paper as channel materials in organic electrochemical transistors and evaluated them as enzyme-free hydrogen peroxide sensors, achieving a limit of detection of 0.79 µM and a sensitivity of 8.5% per decade, highlighting the potential of combining fiber-wall engineering with scalable processing and device integration.